Environmental Engineering Reference
In-Depth Information
Methanosarcinales species within Unit III sediments are indicative for the
non-sulfidic freshwater palaeo-Ace Lake. About 9400 calendar years BP, the
post-glacial sea-level rise resulted in the introduction of sulfate-containing
marine waters into Ace Lake. Low levels of carotenoids and a 16S rDNA
of a
Chlorobium
species (99.6% sequence similarity to
C. phaeovibrioides
DSMZ 269
T
) were detected. This proves that anoxygenic photosynthesis by
Chlorobiaceae, which oxidize sulfide to sulfate, developed soon after Ace Lake
became a fjord system. Lipids indicative for methanogenic Archaea disappeared
and anaerobic bacterial sulfate reduction to sulfide became involved in the active
cycling of sulfur.
The reappearance of lipid biomarkers of methanogenic Archaea and the
concomitant presence of a sterol of ancient aerobic methanotrophic bacteria of
the family Methylococcaceae in the upper 39 cm of the core indicate that an
active methane cycle developed 3000 years BP. The onset of methanogenesis
most likely reflects a concomitant depletion in bottom water sulfate. Sulfate
depletion was most likely a result of turn-over events and the venting of gaseous
H
2
S to the atmosphere, indicated by the marked decrease in carotenoid content
in the top sediment layers.
The ratio between DNA to chlorobactene of the predominant
Chlorobium
species revealed that a substantial part of the DNA of dead but intact cells was
degraded before burial in the sediment. Within the anoxic, sulfidic, Holocene
sediments of Ace Lake, the remaining DNA is well preserved and forms a
species-specific molecular record of microorganisms which colonized the an-
cient water column. In contrast, within the Unit II sediment layers older than
5,700 years, a substantial part of the intact carotenoid chlorobactene of GSB was
reduced to chlorobactane and was therefore less stable than DNA. Since anoxic
settings with potential preserving conditions for ancient DNA can be found
throughout the world, the combined stratigraphic analysis of lipid biomarkers
and ancient 16S rDNA is a promising and powerful tool for reconstructing the
palaeomicrobiology of aquatic systems as well as to refine the reconstruction
of palaeoenvironments and important biogeochemical processes. Nevertheless,
a more detailed analysis on the impact of H
2
S concentration and residence time
of cells within the water column is still needed to improve our knowledge of
the preservation of ancient fossil DNA.
References
[1]
Benson D.A., Karsch-Mizrachi I., Lipman D.J., Ostell J., Rapp B.A. and Wheeler D.L.
GenBank. Nucl Acids Res 2000; 28:15-18.
[2]
Bird M.I., Chivas A.R., Radnell C.J. and Burton H.R. Sedimentological and stable-
isotope evolution of Ace Lake in the Vestfold Hills, Antarctica. Palaeogeo Palaeoclimatol
Palaeoecol 1991; 84:109-130.
